TLS question

Hello:
I am trying to get TLS working on openldap-2.3.20. when I initiate a
search, the debug info at the server indicates "unknown_ca". According to
RFC 2246, this means that the "CA certificate could not be located or
couldn't be matched with a known, trusted CA". My question: Isn't the
slapd.conf "TLSCACertificateFile" directive what tells slapd which CA to
trust? If so, why isn't it working?
thanks